Hank Green: Hi! Today we're gonna continue talking about one of the four fundamental forces of physics, the strong force, which is so awesomely strong that it's taking two episodes for me to describe it to you, so if you haven't already, I highly suggest watching the first one. (I don't know what's up with this [helium animal balloon]. Don't worry, we'll both be here when you get back.) [intro music] Now, hopefully you can tell by its name that the strong force is strong, but how powerful is it? Well, it's powerful enough to make this [floating balloon] happen. The fact that helium exists, or even, y'know, that any atoms exist, is a pretty big deal, and it's important to understand why. You look at an atom of helium. Its nucleus is made of two protons and usually two neutrons, and the thing is, those protons hate the living crap out of each other, like, more than anything else in the universe. Those protons want to get away from each other because they are both positively charged. And yet they exist together in the tiniest space you could probably think of, and they do it because of strong force, the same force that holds the quarks together to make these neutrons and protons. But the brand of strong force that's working between the protons and the neutrons within a nucleus is called the nuclear force, and it's important for two reasons. First, it's strongly repulsive at very short distances, so nuclear force keeps protons and neutrons away from each other a bit, even though neutrons are electrically neutral and don't care who they hang out with. This repulsion helps give atomic nuclei their size. The second reason nuclear force is important is because it's strongly attractive at slightly larger distances -- up to about a quadrillionth of a meter. So, the neutrons and the protons are being kept apart and bound together by the same force at the same time. This strong force interaction is powerful stuff, which is why so much energy is released when atoms are split. Nuclear force is seen as a residual force, brought on by the even stronger version of the strong force, called color force, that I talked about in the last video, that binds quarks together within protons and neutrons. Particles like neutrons and protons that are made up of quarks are called hadrons. As you'll recall, color force is conveyed by force carriers called gluons, which fly around holding quarks together inside a hadron. There's so much energy involved in this activity that it basically forms another kind of force carrier outside of the hadron, called a pion. Pions themselves are made of quarks. They're made of two quarks, and remember that quarks are always shifting between quantum states that physicists call colors. They do this by swapping gluons, a process that holds the quarks together. Now, since color force is being exerted within the pions, and the pions are being exchanged between protons and neutrons, the pions essentially make it possible for color force to be exerted between protons and neutrons. So, keeping it straight, quarks within the protons and neutrons are changing colors as they swap pions from outside, just as they do when passing gluons inside. The main difference is that pions transmit a scaled-down version of strong force. This is totally frickin' weird because, strictly speaking, hadrons shouldn't participate in color interaction, so a pion is like a bus that drives a high school football team from one county to the next, facilitating a game that would never happen otherwise, except in this case the game, instead of being a bunch of people running into each other in pads, is keeping the entire universe together, and helping make sure that this [helium balloon] can happen. Yeah. And yes, I could have used anything as an example here, anything made of atoms. I could've used my fingernail, but I wanted a whale balloon! Thank you for watching this episode of SciShow -- God, get out of my face! I know this isn't technically a whale, sorry, but leave the comment anyway telling me, "It's not a whale!" If you have any questions or comments or ideas, please leave those in the comments below or on Facebook or Twitter, and we will see you next time. [outro music]